This invention relates to programmable logic devices and more particularly to sense amplifier circuits for use in a programmable logic device, and in particular to sense amplifier circuits for use in a programmable logic device that are connected to a power down circuit.
Programmable logic devices are user configurable integrated circuits. Examples of such are provided in a hand book/data book published in 1986 by Advanced Micro Devices, Inc., and entitled "Programmable Logic Hand Book/Data Book".
The programmable logic devices contain a set of undedicated inputs, a set of undedicated outputs and a programmable array which permits the logic connection of these inputs to a given output. A simplified logical representation of a prior programmable logic device is illustrated FIG. 1.
The undedicated inputs IN.sub.1 through IN.sub.N are the vertical line inputs and are normally called rows. The horizontal lines are called product terms and are indicated as PT.sub.1 through PT.sub.3. Each product term is connected to the programmable array which may be programed with any of the standard form of memory elements such as mask programmable shunts, fusable links, EPROM cells, EEPROM cells, SRAM cells, DRAM cells, etc. Controlling the state of the different cells within the programmable array can enable the user to determine which of the inputs are to be included in the combining function on each product term.
In any programmable logic device, and particularly the more complex logic devices, the probability of a given application utilizing all of the available product terms on each output is very low. As a result, some programmable logic devices such as that shown in FIG. 1 contain a mechanism to either disable the unused product term in a given design, or disable the extra product term in a more complex programmable logic device such that it emulates a smaller or simpler programmable logic device.
The programmable logic device of FIG. 1 contains an additional row called the product term disable row, PTD. In normal operation the PTD is always selected. If all the product terms are not attached to a given output then one of the programmable switches 8, 9 or 40 is programed to close and thus disable that unneeded product term. This forces the output of a connecting sense amplifier 11 to a fixed logic state independent of the other inputs (such as a don't care state) so as not to adversely affect the output gate 12. The disadvantage with the product term disable on the configuration shown on FIG. 1 is that even an unused output sense amplifier 11 draws or consumes power and is thus uneconomical to use from a power dissipation point of view.